Pros: power, build quality, balance, low volume channel matching, excellent gain system
Cons: noise floor
This is based on a comparative review of the Rx MKIII-B and Rx MKIII-B+ I recently finished.
I've been one of the biggest Rx fans since 2009. ALO's experience has grown since then, and the high-end market has blossomed into a hi-def, balanced one. RXIII-B carries the Rx tradition of excellence forward. Rx still is one of the best all-round portable amps out there. But today's MKIII-B+ more than one-ups it in power, battery life, and connection quality/options. Power-wise, it is enough to push LCD-2s to semi-dangerous levels without distortion. Even whilst pushing them into dangerous volume levels, the RX MKIII is able to keep incredible signal quality going.
What is amazing is that the B+ version's gain control, which is one if not the best implementation of gains I've ever seen. Despite being able to push the LCD-2 to dangerous levels, it has control and finesse for sensitive IEMs and headphones. Previously, Rx MKIIs and early MKIIIs put out too much voltage into low gain levels, making some earphones/headphones too loud for sensitive-eared people. RX MKIII B+ matches the requirements for ever IEM I have tried, including the uber-sensitive Sleek Audio CT7, which can howl at certain amps' lowest volume settings.
I had missed the original Rx digital volume because perfect balance with sensitive phones was impossible with later versions. RX MKIII-B+ is perfect again. Which makes enjoying that detailed, clean Rx sound all the better.
Seems to go forever. I listen to portable amps whilst on the train going from shoot to shoot, or whilst editing photos. Admittedly I don't listen as much as some others do, I have earphones in my ears for hours a day. And typically, the RX MKIII-B+ lasted about a week before I needed to re-charge it. Amps with this much power and resolution generally have terrible battery life. I've been very impressed.
The Rx tradition of resolution over sound signature continues. This amp spits out perfectly flat signals from end to end. Mid are somewhat preferred when you get to sussing the stereo image. Lots and lots of midrange detail. And it is very open, and airy. But crisp edge delineates everything that hits the ear. There is no blurring, no sizzle, no softness. Detail-lovers will love this sound. Classical, trance and live music are particularly energetic, keen, wide, and airy.
Bass is controlled by about 4,5dB. Enacting it lowers overall volume by 1dB and then amplifies bass above that by 3,5dB for a total of ~4,5dB. 4,5dB isn't going to satisfy bass heads. The adjustment hits lower frequencies to very slightly upper bass levels. Its roll off is perfectly timed to not interfere in the mids. Because it isn't overly aggressive, keeping it on won't ruin any music, and is just enough to warm up or emphasise the lows. I think this approach is very good.
The only fly in the ointment is background noise. It's been a mark of Rx amps since the beginning. ALO have made amps with very very low noise floors. The International is it. I really really wish that amount of low noise could hit the Rx series. Now that Rx has returned to perfect IEM/portable headphone control, it needs to. While this achilles heel isn't overly large, it is obvious. As a series, Rx has pretty much been perfected. It has returned to the perfect balance of the original and so, it works perfectly with everything from current-shy high-ohm balanced headphones to current-hungry cans and sensitive earphones in any mode. But if you primarily listen to sensitive IEMs (108-120dB), Rx isn't my first recommendation. It should be. It's got the power, the control, the resolution and the battery life to make portable listening perfect. But background noise is still too high.
The Rx series sort of went off on its own way for a while. It never strayed from excellence, but the first version was my favourite for earphone use because of its excellent left-right balance with sensitive earphones. MKIII-B+ can finally be used as well as the original can for the same earphones. And it does balanced output/input as well, so there is lots of room for experimentation. Output quality has been improved and there is more power than before. And better battery life. It is the series' best upgrade and yet it is a return to the series' excellent start. The only thing Rx needs is a lower background noise output. ALO already proved that they can do that. Still, this is one of the best, if not the best high-resolution portable amp out there for ALL your headphone needs. There are few portable amps that offer as much power as this Rx and yet are still able to control earphones just fine.